Hitherto, as filament materials used for industrial materials, agricultural materials, fisheries materials, etc, particularly as fishing nets and fishing lines, there have been used synthetic resins, such as polyamide, polyester, polyolefin, polyvinylidene chloride and polyvinylidene fluoride, in view of mechanical properties required thereof. However, synthetic resin filaments composed of such synthetic resins are hardly decomposed in natural environments, so that if they are thrown away or left as they are, they remain semipermanently in the nature, thus posing a revere problem in environmental by genies. Particularly, there occur increasing troubles that discarded fishing nets and cut fishing lines are accumulated at the bottoms of the sea or lakes, and birds and creatures in water are entangled therewith to be killed or injured. Accordingly, an improvement in this respect is seriously desired from the viewpoints of environmental preservation and protection of the nature.
For this reason, in recent years, there have been frequently made development works regarding biodegradable filaments, for use as fisheries materials, such as fishing lines, fishing nets and fanning nets, or those used as agricultural materials and industrial materials that are biologically degraded after their actual use (Patent documents 1 and 2 below).
Further, biodegradable filaments are also used as polymer materials for medical use, such as biologically absorbable suture for physical surgery and artificial skins (Patent documents 3 and 4 below).
However, none of hitherto available biodegradable filaments have high mechanical strength and high biodegradability. Particularly, as for filaments for fishing lines, etc., knot strength is thought most of since they are frequently used in a knotted state, whereas no biodegradable filaments have satisfied a tensile strength of at least 750 MPa and a knot strength of at least 600 MPa which are minimum levels of high-strength filaments, such as those of polyamide, polyester and polyvinylidene fluoride, and a tensile elongation of 10-50% which is not too high or not too low in view of practical properties, such sensitivity, impact-absorptivity and handleability.
For satisfying these practical properties, biodegradable filaments having a core-sheath structure comprising a combination of different resins for the core and the sheath have been proposed (Patent documents 2 and 5 below), whereas none of them have satisfied the above-mentioned practical properties. For example, the composite filament of Patent document 2 has exhibited a tensile strength of ca. 739 MPa (6.6 g/denier) at the maximum and a knot strength of 615 MPa (5.5 g/denier) at the maximum, and the composite filament of Patent document 5 is described to exhibit a maximum tensile strength of 1000 MPa but also exhibited too large a tensile elongation of 70-250%.
Patent document 1: JP-B 2779972
Patent document 2: JP-A 10-102323
Patent document 3: U.S. Pat. No. 3,297,033
Patent document 4: JP-B 58-1942
Patent document 5: JP-B 3474482